Typical elevator door operating mechanisms, such as that shown in commonly owned U.S. Pat. No. 4,305,481, employ a rotary motor which drives a complex lever mechanism. In attempts to simplify, and thereby reduce the initial cost, installation, adjustment and maintenance costs, the use of linear induction motors has been suggested as in U.S. Pat. No. 3,891,907 and in commonly owned copending U.S. patent application Ser. No. 029,203 filed Mar. 10, 1993. The use of a linear induction motor eliminates the need for any mechanical parts other than an electrically conductive secondary to which the door may be affixed, and which will drive the door in the open and closed direction in dependence upon currents applied to the windings of the linear induction motor primary. Utilizing a digital computer for generating the variable phase motor current waveforms for driving a variable voltage, variable frequency linear induction motor results in waveforms which are only approximations of the pure sinusoidal waveforms that produce optimum linear induction motor performance. Distortion in digitally-produced waveforms result in acoustic noise and increased motor heating. In the art, it has been known to utilize advanced techniques to improve the current drives by means of closed loop current control implemented through digital signal processing. However, closed loop current control significantly increases the complexity of the processing as well as the cost of the apparatus itself.
As described in the aforementioned U.S. patent application, the elevator door is caused to rise about 0.2-0.3 millimeters as a result of the normal forces induced between the linear induction motor primary and secondary. While this seems very small, it is visually perceptible and is accompanied by a pronounced "thump". It may also induce secondary effects which are also audibly perceptible.